Frontiers in extreme nonlinear optics: Attosecond-to-zeptosecond coherent kiloelectronvolt X-rays on a tabletop

Summary form only given. The past three years in a row marked the 50th anniversaries of three significant innovations in optics: the invention of the laser; the discovery of the nonlinear upconversion of laser light in a spectral region where laser light has not been available; and the outlining of phase matching of this upconversion process - a recipe that makes the newly generated laser-like light bright and usable for applications. The same revolution that made it possible to create well directed beams in the visible region of the spectrum is only now happening for X-rays. Large-scale X-ray free electron lasers are promising to capture images of ultrafast dynamics in a single shot. An extreme version of nonlinear optics - high harmonic generation (HHG) - can also generate bright, coherent, beams of X-rays, with very short wavelengths <;7.7 angstroms, in a tabletop-scale setup for the first time [1]. This practically realizes a coherent version of the Roentgen X-ray tube in the soft X-ray region. Improved understanding of the microscopic quantum physics and macroscopic nonlinear optics of high harmonic generation [2-5], as well as the development of novel ultrafast mid-IR lasers [6] have lead to this rapid progress in the past few years, essentially solving the phase matching problem of HHG in the X-ray region. In addition, these kiloelectronvolt HHG X-rays have a supercontinuum structure with the broadest coherent bandwidth (>1.3 keV) that any light source, large or small scale, can generate to date. Such an ultrabroad spectral bandwidth can support X-ray pulses as short as 2.5 attoseconds and is scalable towards zeptosecond pulse durations. These unique, ultrafast, laser-like X-ray beams promise revolutionary new capabilities for understanding and controlling how the nanoworld works on its fundamental time and length scales. This understanding is relevant to the next generation data and energy storage devices, nano-electronics, bioimaging, and future medical diagnostics.